In electrical apparatus, such as salient pole dynamoelectric machines, it is necessary that the rotor, possibly weighing up to several hundred tons or more, be near perfectly balanced about its axis of rotation; viz, within a couple of ounce - inches. A typical rotor is illustrated in U.S. Pat. No. 4,358,968 to Peterson et al.
The balancing process begins with the manufacture and assembly of the rotor components. The rotor is forged within a very tight tolerance. Conductor windings, laminations and rotor bars are weighed to assure that all symmetrical components are virtually identical in weight. For example, one pole winding would be manufactured to weigh exactly as much as each of the other pole windings. However, with an apparatus as large as a dynamoelectric salient pole rotor, it is impossible to perfectly balance the rotor about its axis of rotation by precision manufacturing and assembly alone.
The object of rotor balancing is to prevent excessive vibrations at the rotor bearings and to allow smaller clearance between the rotating and stationary parts of the machine. Aerodynamically and electrically, it is ideal to minimize this distance (i.e., windage losses). However, from a safety standpoint, clearance between the stationary and rotating components (including smaller clearances caused by vibration) must be provided. Furthermore, the more perfectly balanced the rotor, the lower are the rotational losses. Therefore, since the requisite rotor balancing cannot be accomplished by manufacturing all subcomponents to a tight tolerance and assembling all subcomponents with precision, additional balancing techniques must be employed.
One technique of balancing the rotor is by affixing weights on the rotor periphery. In this manner, the assembled rotor can be both statically and dynamically balanced. Those skilled in the art know that rotor balancing involves the arrangement of weights in two planes. Preferably the balancing weights should be few and as small in size as possible, thus making residual imbalance as small as possible.
Although, rotor balancing must be kept in mind during construction of the rotor, it is often impossible to predict the preferred balancing locations. Therefore, the most useful rotor design is one that provides flexibility of rotor balancing locations and one that allows for the use of many combinations of relatively small balancing weights.
Salient pole machines are often designed without primary regard to the difficult problem of rotor balancing. They do allow for balancing in two planes; they do not allow for balancing about the quadratures axis of each plane, where the quadrature is the axis of symmetry between the midpoint of the field poles. This inability to balance the rotor in each plane about the direct and quadrature axes has necessitated the use of large balancing weights which cause greater rotational stresses.
Clearly, a salient pole rotor design which provides for the addition of many combinations of small balancing weights about the direct and quadrature axes, one which could be easily adapted to existing generator designs and one which is relatively inexpensive would be widely accepted by the industry and would go far toward achieving optimum rotor design.